i have exactly the same laptop and had the same problem but only with the nvidia drivers and the Fn keys are not working

This is what i did to fix this problem
First i did try setting the backlight level without the Nvidia driver this worked fine without any problems with the vesa drivers and nv. I can only set this trough a app for gnome , gnome brightness aplet and for kde the Kpowersave tool. The Fn key's are not working in my case.

If this is working install the Nvidia driver.
You can do this in most distro's easily with synaptics package manager. If installed you need to get the config right. You can do this with the command: sudo nvidia-xconfig.

At last you need to edit the file /etc/X11/xorg.conf
in the section device you need to add this: Option "RegistryDwords" "EnableBrightnessControl=1"